Print PageBecause soil C cycling is ultimately the consequence of microbial growth and activity, the mechanistic basic for understanding C decomposition, transformation and stabilization in soils lies in a detailed understanding of general microbial physiology and process. It has been well established that microorganisms play a critical role as the “gatekeepers” for terrestrial C fluxes, either causing its release to the atmosphere through their decomposition activities, or preventing its release by stabilizing the C in a form that cannot be easily decomposed. A substantial amount of research has been focused on microbial sources of carbon-gas production, but much less attention has been paid to role of microbial communities in C sequestration. In this study we investigated the microbial role as carbon “stewards” in ecosystems, focusing primarily on the production of recalcitrant carbon under varying global change conditions at the Jasper Ridge Global Change Experiment, Stanford, CA. We quantified amino sugars in surface soils (0-10cm) from the California annual grassland ecosystem following nine years of exposure to elevated CO2, warming, and nitrogen (N) deposition, alone or in combination.
Results/Conclusions
We found that CO2 was not a direct factor influencing soil amino sugar and soil C pools, while the amino sugar:total carbon (AS/TC) ratio significantly declined after exposure to evaluated temperature and elevated N conditions. This indicates that warming and N deposition result in significant decrease in microbial contributions to soil C pool. In another words, microbial residue-C in soils are preferentially depleted compared to general soil C under those two scenarios. There appear to be two mechanisms operating to explain the decline in AS/TC ratio. For the warming treatment, amino sugars decreased substantially and soil total C was relatively constant; while under N deposition, soil total C increased and amino sugars decreased slightly. The AS/TC ratio decline was driven by two distinct factors: temperature and nitrogen, but with similar effect. This implies two mechanisms at play in determining the stabilization versus turnover of microbial carbon, and indicates that caution may be necessary when interpreting results of simple or single measures of microbial communities. We suggest that the apparent sensitivity of microbial residues to temperature or nitrogen may have implications for our predictions of global change impacts on soil stored C. The “Microbial Carbon Pump” (MCP) recently developed by marine researchers may be of use for understanding terrestrial C fixation. The decrease in microbial C contribution observed in our study indicates a malfunction of the MCP under warming and N deposition in soils that may have implications for long-term soil and climate carbon balances. The microbial community, by driving the terrestrial microbial carbon pump, acts as a critical climate steward.
